Unpack format characters in Python

Question

I need the Python analog for this Perl string:

unpack("nNccH*", string_val)

I need the nNccH* - data format in Python format characters.

In Perl it unpack binary data to five variables:

• 16 bit value in "network" (big-endian)
• 32 bit value in "network" (big-endian)
• Signed char (8-bit integer) value
• Signed char (8-bit integer) value
• Hexadecimal string, high nibble first

But I can't do it in Python

More:

bstring = ''
while DataByte = client[0].recv(1):
    bstring += DataByte
print len(bstring)
if len(bstring):
    a, b, c, d, e = unpack("nNccH*", bstring)

I never wrote in Perl or Python, but my current task is to write a multithreading Python server that was written in Perl...

Answer 1

The Perl format "nNcc" is equivalent to the Python format "!HLbb". There is no direct equivalent in Python for Perl's "H*".

There are two problems.

• Python's struct.unpack does not accept the wildcard character, *
• Python's struct.unpack does not "hexlify" data strings

The first problem can be worked-around using a helper function like unpack.

The second problem can be solved using binascii.hexlify:

import struct
import binascii

def unpack(fmt, data):
    """
    Return struct.unpack(fmt, data) with the optional single * in fmt replaced with
    the appropriate number, given the length of data.
    """
    # http://stackoverflow.com/a/7867892/190597
    try:
        return struct.unpack(fmt, data)
    except struct.error:
        flen = struct.calcsize(fmt.replace('*', ''))
        alen = len(data)
        idx = fmt.find('*')
        before_char = fmt[idx-1]
        n = (alen-flen)//struct.calcsize(before_char)+1
        fmt = ''.join((fmt[:idx-1], str(n), before_char, fmt[idx+1:]))
        return struct.unpack(fmt, data)

data = open('data').read()
x = list(unpack("!HLbbs*", data))
# x[-1].encode('hex') works in Python 2, but not in Python 3
x[-1] = binascii.hexlify(x[-1])
print(x)

When tested on data produced by this Perl script:

$line = pack("nNccH*", 1, 2, 10, 4, '1fba');
print "$line";

The Python script yields

[1, 2, 10, 4, '1fba']

Answer 2

The equivalent Python function you're looking for is struct.unpack. Documentation of the format string is here: http://docs.python.org/library/struct.html

You will have a better chance of getting help if you actually explain what kind of unpacking you need. Not everyone knows Perl.